Project Fi is Google's cell phone service. Base amount is $20 and $10 per GB. They currently act as a "prepaid" phone service. Meaning, you pay in the beginning of the month for the allotted amount of data you would like. The best part is that you are reimbursed for whatever you don't use. For example, if I have a 3GB plan and i only use 2.1GB you will be give back $9!! Google utilizes Tmobile, Sprint and US Cellular as their network coverage. Though these alone can be spotty at times, together they are GREAT! The intelligent SIM bounces between these networks as well as secure, high speed wifi networks. All this is done without the user even realizing it.
The Catch....there really isnt one besides the fact you can only at the moment use the Nexus 5, 5x and 6p. Which are all great phones anyways. End of the day also if you are not into the plan you have an awesome unlocked phone!
End of the day its awesome and I would I highly recommend it!